Anatomical picture of the flat rachitic pelvis
Bone deformation caused by past rickets leads to specific changes in proportions. The main feature is that the sacral promontory strongly projects into the cavity. Because of this, the inlet plane acquires a characteristic heart shape, and its anteroposterior dimension is shortened.
In pronounced deformations, the classical ratio of transverse dimensions is disrupted: the distance between the anterior superior iliac spines becomes greater than the distance between their iliac crests. The wings of the ischial bones flare out, causing their tuberosities to deviate significantly outward. The pubic arch becomes wider compared to normal structure.
Interestingly, the narrowing predominantly affects the inlet: the anteroposterior and transverse dimensions of the pelvic cavity itself remain normal or even increase. The dimensions of the outlet plane are also increased. Due to these volume differences, the pelvic axis is deformed and represents a broken line.
Diagnostics and pelvimetry
The detection of flat rachitic deformation is based on history taking and physical examination. Suspicion should arise if there are traces of rickets suffered in childhood: "square" head shape, curvature of the lower limbs, spine, or sternum.
Key pelvimetric and anatomical markers:
- Reduction of the difference between distantia cristarum and distantia spinarum.
- Shortening of the conjugata diagonalis.
- Reduction of the vertical dimension of Michaelis rhombus.
Vaginal examination reveals a flattened sacrum deflected posteriorly. A characteristic sign is the presence of a "false promontory" — not only the true promontory projects into the cavity, but also the 2nd sacral vertebra. The coccyx is hooked inward.
Obliquely displaced (asymmetric) pelvis
This is one of the rare, but clinically significant forms of contracted pelvis. The etiology is associated with pathologies that disrupt the symmetry of the musculoskeletal system in childhood. These include scoliosis, coxitis, unilateral hip dislocation, shortening of one of the lower extremities, as well as poorly healed pelvic and leg fractures or functional disorders of one leg.
The pathogenesis of the deformation is based on the uneven distribution of body weight. The torso finds its main support in the hip joint on the healthy side. As a result, the acetabular region of the healthy side is gradually pressed into the cavity. Meanwhile, the pelvic half on the side of the affected leg becomes narrower.
Obstetrical management in asymmetry
Despite pronounced deformation, an obliquely displaced pelvis is not always an insurmountable obstacle to spontaneous vaginal delivery.
Narrowing of one side is often compensated by the fact that the opposite half remains relatively spacious. Due to this, this form may not hinder the normal biomechanics of labor and fetal head descent. An important role is played by the woman's behavior: during labor, she instinctively adopts the body position that is most biomechanically advantageous for the fetus to pass through this specific anatomical situation.
Briefly: answers to topical questions
What are the features of labor biomechanics in a flat rachitic pelvis?
The features of labor biomechanics in a flat rachitic pelvis are due to the reduction of the anteroposterior dimension of the inlet plane. The engagement of the head is characterized by the following triad of signs:
- Prolonged standing — the head stays above the inlet for a long time, engaging with the sagittal suture in the transverse dimension.
- Head extension — slight extension allows the head to pass through the narrowed anteroposterior dimension via a small transverse dimension (8.5 cm).
- Asynclitic engagement — extra-axial engagement, most commonly anterior asynclitism, where the head engages with the anterior parietal bone while the posterior bone rests against the sacral promontory (promontorium).
After passing the inlet, the head flexes. Further labor may acquire a "stormy" character due to the relatively increased anteroposterior dimensions of the pelvic cavity and outlet.
How do the shape and dimensions of Michaelis rhombus change in a flat rachitic pelvis?
In a flat rachitic pelvis, the upper half of Michaelis rhombus is flattened, the upper angle becomes obtuse, and the longitudinal (vertical) dimension is significantly reduced.
To remember the signs: Sacrum is flattened, promontory sticks out, rhombus is shortened — look for rickets. This combines the main bone deformations and the amnestic cause of narrowing.
Class and exam questions
How do the dimensions of the cavity and outlet change in flat rachitic deformation?
Unlike the inlet plane, the anteroposterior and transverse dimensions of the pelvic cavity remain normal or even increase. The dimensions of the outlet plane are also increased.
What is a "false promontory"?
This is a specific anatomical sign in which not only the true promontory, but also the body of the 2nd sacral vertebra protrudes into the pelvic cavity.
Which half of the pelvis narrows in obliquely displaced deformation?
The half on the side of the affected leg narrows, although the main support of the torso and the indentation of the acetabulum occur on the side of the healthy limb.
What else to study on this topic
- Kyphotic pelvis and its effect on labor biomechanics
- Deformations caused by exostoses and bone tumors
- Evaluation of Michaelis rhombus in pelvimetry
- Instinctive postures of the parturient woman in asymmetric pelvic contraction
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